1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
6#define COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
7
8#include <functional>
9#include <string>
10#include <vector>
11
12#include "base/basictypes.h"
13#include "base/compiler_specific.h"
14#include "base/metrics/field_trial.h"
15#include "third_party/mt19937ar/mt19937ar.h"
16
17namespace metrics {
18
19// Internals of entropy_provider.cc exposed for testing.
20namespace internal {
21
22// A functor that generates random numbers based on a seed, using the Mersenne
23// Twister algorithm. Suitable for use with std::random_shuffle().
24struct SeededRandGenerator : std::unary_function<uint32, uint32> {
25  explicit SeededRandGenerator(uint32 seed);
26  ~SeededRandGenerator();
27
28  // Returns a random number in range [0, range).
29  uint32 operator()(uint32 range);
30
31  MersenneTwister mersenne_twister_;
32};
33
34// Fills |mapping| to create a bijection of values in the range of
35// [0, |mapping.size()|), permuted based on |randomization_seed|.
36void PermuteMappingUsingRandomizationSeed(uint32 randomization_seed,
37                                          std::vector<uint16>* mapping);
38
39}  // namespace internal
40
41// SHA1EntropyProvider is an entropy provider suitable for high entropy
42// sources. It works by taking the first 64 bits of the SHA1 hash of the
43// entropy source concatenated with the trial name and using that for the
44// final entropy value.
45class SHA1EntropyProvider : public base::FieldTrial::EntropyProvider {
46 public:
47  // Creates a SHA1EntropyProvider with the given |entropy_source|, which
48  // should contain a large amount of entropy - for example, a textual
49  // representation of a persistent randomly-generated 128-bit value.
50  explicit SHA1EntropyProvider(const std::string& entropy_source);
51  virtual ~SHA1EntropyProvider();
52
53  // base::FieldTrial::EntropyProvider implementation:
54  virtual double GetEntropyForTrial(const std::string& trial_name,
55                                    uint32 randomization_seed) const OVERRIDE;
56
57 private:
58  std::string entropy_source_;
59
60  DISALLOW_COPY_AND_ASSIGN(SHA1EntropyProvider);
61};
62
63// PermutedEntropyProvider is an entropy provider suitable for low entropy
64// sources (below 16 bits). It uses the field trial name to generate a
65// permutation of a mapping array from an initial entropy value to a new value.
66// Note: This provider's performance is O(2^n), where n is the number of bits
67// in the entropy source.
68class PermutedEntropyProvider : public base::FieldTrial::EntropyProvider {
69 public:
70  // Creates a PermutedEntropyProvider with the given |low_entropy_source|,
71  // which should have a value in the range of [0, low_entropy_source_max).
72  PermutedEntropyProvider(uint16 low_entropy_source,
73                          size_t low_entropy_source_max);
74  virtual ~PermutedEntropyProvider();
75
76  // base::FieldTrial::EntropyProvider implementation:
77  virtual double GetEntropyForTrial(const std::string& trial_name,
78                                    uint32 randomization_seed) const OVERRIDE;
79
80 protected:
81  // Performs the permutation algorithm and returns the permuted value that
82  // corresponds to |low_entropy_source_|.
83  virtual uint16 GetPermutedValue(uint32 randomization_seed) const;
84
85 private:
86  uint16 low_entropy_source_;
87  size_t low_entropy_source_max_;
88
89  DISALLOW_COPY_AND_ASSIGN(PermutedEntropyProvider);
90};
91
92}  // namespace metrics
93
94#endif  // COMPONENTS_VARIATIONS_ENTROPY_PROVIDER_H_
95